Conventionally, seamless belts are used in various applications in electrophotographic image forming apparatus. Particularly, in recent full-color electrophotographic image forming apparatus, an intermediate transfer belt is used, on which four developed yellow, magenta, cyan and black images are overlapped and the overlapped images are transferred onto a transfer medium such as a paper at a time.
Four (color) image developers have been used for one photoreceptor when using the intermediate transfer belt, which had a disadvantage of low printing speed. Therefore, in high-speed printing, a train-of-four tandem method continuously transferring each color to a paper with four (color) photoreceptors. However, this is difficult to adjust positional preciseness of overlapping each color due to paper quality, etc., resulting in production of color-shifted images. Then, the train-of-four tandem method has mostly used the intermediate transfer belt recently.
The intermediate transfer belt is also required to satisfy transfer at higher speed and positional preciseness. Particularly, it is required to prevent deformation such as elongation after continuously used for the positional preciseness. Further, the intermediate transfer belt is located over a wide range of the apparatus and required to have flameproofness because of being applied with a high voltage for transfer. In compliance with these requirements, a polyimide resin having high elasticity and high heat resistance is mostly used as a material of the intermediate transfer belt.
Recently, even a full-color electrophotographic image forming apparatus is being required to have high speed printing capability, and high durability and stability. In compliance with the high speed printing capability and high durability, a large apparatus is driven at high speed, and even the intermediate transfer belt needs to have longer circumferential length and to be driven at high speed.
Such an enlarged intermediate transfer belt and a system using the belt have other problems the conventional belt/system do not have. One of them is travelling stability of the intermediate transfer belt. Specifically, the intermediate transfer belt slips on a drive roller driving the belt because of rotating at high speed, resulting in production of color-shifted images, or the belt is likely to be damaged when shifted.
Another problem is uneven properties of the belt. One belt occasionally has uneven properties according to its positions, resulting in production of images having uneven quality. A mould a resin solution is coated on is heated and dried/crosslinked to prepare a typical polyimide belt. A very large mold used to prepare a large belt having a circumferential length not less than 2,000 mm is difficult to uniformly control the temperature of the whole mold, and thought to have uneven properties.
A further problem is a white spot a toner is not transferred onto. When a transfer bias is controlled by constant current control, white spots tend to occur in an environment of low temperature and low humidity, on a backside in both side printing, on a paper having high resistivity, and when the transfer bias is high. The apparatus having high linear speed needs to apply a high voltage to the belt travelling at high speed to pass a transfer nip in a shorter time, which is thought to cause white spots.
Another problem of the intermediate transfer belt is shape and size stability against environmental variation.
The belt tends to curl due to the environmental variation.
The intermediate transfer belt is formed of an endless belt extended by plural rollers with tension. The belt is applied with a predetermined tension by tension rollers and the tension is applied thereto even when remaining still. Therefore, when the belt is left for hours without running, a part thereof supported by the roller is curled. Then, the belt deteriorates in runnability, a toner image transferred onto the curled part is poorly transferred onto a paper, resulting in abnormal images such as stripe images. Further, the belt needs to have size stability when absorbing moisture, particularly a large belt largely varying in size because of having a long circumferential length.
Japanese published unexamined application No. 2008-225182 discloses a polyimide belt having an inner surface roughness (Ra) of from 0.15 to 0.6 μm and a maximum surface roughness of from e to 15 μm to prevent slidability thereof and abrasion powder. However, the belt has a conventional size and possibly has insufficient runnability and other various troubles when enlarged.
Japanese published unexamined application No. 2005-74914 discloses a method of preparing a tubular material without uneven temperature. An apparatus including a cylindrical mold placing a heat pipe including a hollow where a heat medium is circulated on its circumferential wall, and an electromagnetic induction coil electromagnetically heating in the mold is used. Even such an apparatus has uneven temperature.
Japanese published unexamined application No. 2001-142313 discloses a polyamide resin which is a copolymer repeating an A component having an imide bond between a wholly aromatic skeleton which is a tetracarboxyl residue and a p-phenylene skeleton which is a diamine residue, and a B component having an imide bond between the wholly aromatic skeleton which is a tetracarboxyl residue and a diphenyl ether skeleton; and/or a blend mixing a polymer including the A component as a repeat unit and a polymer including the B component as a repeat unit, and which satisfies the following relationship:R≦65−W wherein R represents % by mol of the A component and W represents parts by weight thereof per 100 parts by weight of the polyimide resin which is an electroconductive filler. Such a polyimide resin improves a balance between the flexibility and the rigidity of a belt. However, the backside roughness is not considered and stable runnability is difficult to obtain, and the white spot is not considered at all, either.
Japanese published unexamined application No. 11-282277 discloses an image forming apparatus having a multilayered intermediate transfer belt including a high-resistivity surface layer forming an outer circumferential surface of the belt bearing a toner image and a middle-resistivity base layer forming an inner circumferential surface thereof a transfer bias is applied to. The high-resistivity surface layer has high electrical pressure resistance to prevent the white spot. However, when used in a high-speed machine as a large belt, the belt possibly has problems of runnability and uneven properties because they are not considered.
Because of these reasons, a need exits for a large and high-speed intermediate transfer belt stably running for long periods, having less uneven properties and stable shape and size against the environment, and producing quality images without white spots.